1. Field of the Invention
The present invention relates to a fitting of weldable thermoplastic material. The fitting is used for connecting pipe elements or other fittings. The connection is obtained by welding by means of at least one heating element arranged at the surfaces of the members to be welded together. At least one weld indicator defined by a hollow space in the fitting is provided.
2. Description of the Prior Art
Fittings of thermoplastic material of the above-described type are known. These fittings are welded by means of heating elements which include an electric heating wire and are integrated in the fittings.
The heating elements are, for example, heating coils or heating mats. The structural design of the heated fitting determines the principal parameters of the welded connection, i.e., the electrical energy to be supplied within a period of time by means of a control device and the melt pressure generated during the period of energy supply. If the fitting is a closed welding sleeve, the structural design of the fitting is characterized by the inherent shrinking capability of the fitting. If the fitting is a radially mountable, saddle-shaped fitting of appropriate shape, the structural design of the fitting is characterized by the pressing force to be exerted by means of externally mountable tensioning members. In addition, to determine whether the welded connection is free of problems, devices are advantageously provided in the fittings in the area of the heating elements, the devices having become known in the art as weld indicators.
Swiss Pat. No. 632,078 discloses a fitting of the above-described type. The fitting is a sleeve used for connecting pipe elements. The indicating device for monitoring the welding is formed by a cylindrical recess starting at the outer periphery of the sleeve. The bottom of the recess is located in the vicinity of the heating elements, so that a wall member is provided between heating element and recess. This type of fitting facilitates a direct visual indication of any existing melt. However, this visual indication reacts negatively to external influences, such as, ambient temperature and wind. Under extreme circumstances, melt will flow from the joint area in an uncontrolled manner, so that it possible that no welding pressure or a welding pressure which is too small may exist at certain locations. As a result, it cannot be ensured that the welded connection is free of problems in these areas.
German Pat. No. 2,760,064 discloses a welding sleeve with a weld indicator which includes a sensor pin surrounded by a free space, the base of the pin being arranged at the bottom of the latter. An uncontrolled discharge of melt is not likely in this weld indicator. However, a direct visualization of the melt is normally not possible. This indirect indication by the shifting of the pin beyond the outer periphery of the sleeve is very unsafe due to outer influences and the like. Thus, particularly when the fittings have thick walls, a reliable indication of a problem-free welded connection is not ensured.
An improved weld indicator is disclosed in European application No. 0,173,174. This known weld indicator includes an enclosed hollow space and a piston with an indicator pin arranged in the hollow space. This weld indicator also has the disadvantage that the melt is visualized indirectly. In addition, incorrect indications may occur, for example, due to the formation of gas or vapor pressure. In addition, the shifting of the piston is dependent upon the frictional conditions thereof relative to the hollow space and, thus, also depends upon the dimensional tolerances of the piston and of the hollow space. Therefore, the indication is also unreliable and, furthermore, the indicator is complicated in its design.
It is, therefore, the primary object of the present invention to provide a fitting with a weld indicator of the above-described type, wherein the melt can be made directly visible by means of an indicator which is structurally very simple, and wherein the indicator operates in such a way that the melt cannot flow out in an uncontrolled manner.